The present invention relates to an optical device with a pig tail optical fiber in which an optical fiber is connected to an optical waveguide. The invention relates more particularly to an optical device with a pig tail optical fiber having a optical fiber for mode adjustment which connects an optical fiber with a low specific refractive index difference and an optical waveguide with a high refractive index, and to its production method.
Research and development of a waveguide type optical device using quartz base glass have been actively carried out with the objectives of cost lowering, downsizing and advancing the functions of such an optical device.
FIGS. 6(a) and 6(b) show the constitution of a conventional waveguide type device with a pig tail optical fiber. FIG. 6(a) is a plan view and FIG. 6(b) is a sectional view taken along line A-A' of FIG. 6(a). The device has structure in which a usual single mode fiber 1 is connected to an input terminal of an optical waveguide 22. The reason why a usual single mode fiber is employed as an optical fiber is that using a single mode fiber is indispensable to cost lowering of a waveguide type device with a pig tail optical fiber, since a single mode fiber is cheap, even though a low specific refractive index difference results.
The optical waveguide 22 has a core 63, having a rectangular cross-section with a width w and a thickness t built in a cladding 53 formed on a substrate 7 made from Si or SiO.sub.2. The usual single mode fiber 1 has a core 61 with a diameter a.sub.1 covered by a cladding 51 with an outer diameter b.sub.1. The optical waveguide 22 and the usual single mode fiber 1 are connected by laser welding, for example.
If a specific refractive index difference .DELTA..sub.1 of the core 63 and the cladding 53 in the optical waveguide 22 is different from the specific refractive index difference .DELTA..sub.2 of the core 61 and the cladding 51 in the constitution shown in FIG. 6, it is known that the following problems may occur.
(1) A part of a light signal incident on the optical waveguide 22 from the usual single m,ode fiber 1 is reflected at the connection 21 of the waveguide 22 and the fiber 1, and then the strength of the light signal is reduced. On the other hand, the strength of a light signal incident on the usual mode fiber 1 from the optical waveguide 22 is also reduced. Consequently, a large connection loss results.
(2) The above-mentioned light reflection causes mutual interference between the light signals, which results in degradation of the transmission, such as an increase in cross talk.
(3) If .DELTA..sub.2 &lt;.DELTA..sub.1, the condition that a.sub.1 &gt;t, w is to be satisfied. In that situation, a part of the light signal propagating in the usual single mode fiber 1 leaks from the core 63 of the optical waveguide 22 and propagates in the cladding 53. The light signal propagating in the cladding 53 gets abroad as it propagates in the optical waveguide 22 while repeating multiple reflections between the two terminal faces of the optical waveguide 22, which results in a degradation of light transmission characteristics, such as deterioration of the band characteristics, a rising and descending of the characteristics of the light pulse signal, an increase in cross talk and so on.
Providing a mode adjustment circuit in a waveguide, or applying the mode conjunction by diffusing the dopant for the reactive index distribution control in a core by heat addition to a terminal of an optical waveguide, is adopted in order to prevent the above-mentioned connection loss or characteristics degradation.
However, providing a mode adjustment circuit in a waveguide has the problem that it becomes difficult to downsize the optical device since the size of a waveguide will become considerably large when using the mode adjustment circuit. On the other hand, diffusing the core dopant at a terminal of the optical waveguide seems to avoid the above-mentioned problem; however, this approach raises the following problems.
(1) It is difficult to diffuse the dopant for the refractive index distribution control uniformly in the core, since an optical waveguide does not have a cylindrically symmetric structure as an optical fiber, but has a plate shape asymmetric structure. That is, since the distribution of the added heat is asymmetric, the diffusion of the dopant for the refractive index distribution control in a core becomes asymmetric. Consequently, a complete mode adjustment becomes very difficult, and an undesirable effect of polarization dependency is induced.
(2) In heating an optical waveguide, it is difficult to heat up the core and the cladding layer to the target temperature, since the substrate has a very much larger thickness than the diameters of the core and the cladding, and so a large part of the added heat is absorbed by the substrate.
(3) Furthermore, heating an optical waveguide causes the problem that it changes or degrades the optical transmitting characteristics of the optical waveguide and the other elements in the optical waveguide device, such as an optical branch circuit, an optical star coupler, an optical wave uniting or separating circuit, an optical filter, a ring optical resonance circuit and so on.